(1) Field of the Invention
The present invention relates to ultrasound processing devices and particularly to improvement of elasticity evaluation using ultrasound.
(2) Description of the Related Art
Elasticity evaluation is a technique for acquiring and evaluating a spatial distribution of hard portions/soft portions of internal tissue in a subject, by transmitting ultrasound into the subject. In cancer screening, the importance of early detection of cancer tissue has long been advocated. However, it is very difficult to locate a small amount of cancerous tissue among healthy tissue occupying a large space within a living organism. Thus, various elasticity evaluation techniques using ultrasound processing devices have been established. The following describes shear wave speed (SWS) acquisition by an ultrasound processing device. Evaluation of elastic modulus is possible based on SWS.
A method of SWS acquisition comprises a “shear wave excitation” step, a “shear wave measurement” step, a “tissue displacement detection” step, and a “shear wave propagation analysis” step.
In “shear wave excitation”, a region of interest (ROI) within the subject is defined and ultrasound is transmitted all at once from each of a plurality of acoustic elements constituting an ultrasound probe towards one location in the subject. Ultrasound transmitted all at once in this way is referred to focused ultrasound or an acoustic radiation force impulse (ARFI). According to this focused ultrasound, energy is concentrated in one location in a living organism and a large acoustic radiation force is generated. According to this acoustic radiation force, surrounding tissues receive a shearing force, are displaced, and a transverse wave is generated in a direction parallel to an element array direction of the ultrasound probe. This transverse wave is a “shear wave”.
In “tissue displacement detection”, ultrasound for detection purposes is transmitted from acoustic elements of the ultrasound probe. As ultrasound for detection purposes, a planar pulse may be used, for example. When a planar pulse is used to detect tissue displacement, the state of a wide range of space in a region of interest can be acquired at once. At a predefined sampling rate, the state of pertinent internal tissues is acquired a plurality of times. Subsequently, wavefront positions of a shear wave are derived from displacement between acquired tissue states. Thus, both a region in which the wavefront of the shear wave makes great progress and a region in which the wavefront of the shear wave makes slight progress can be acquired.
Magnitude of changes in wavefront position indicates magnitude of shear wave speed, and the magnitude of shear wave speed squared indicates hardness/softness of internal tissues that reflected a planar pulse. A region in which the shear wave makes a small amount of progress is determined to be healthy, and healthy tissue such as adipose tissue is determined to be present. A region in which the shear wave makes a comparatively large amount of progress is suspected to include cancerous tissue. The shear wave speed obtained by the above processing, when converted to an image of hardness/softness of tissue elasticity, allows visual representation of spatial distribution of hardness/softness of tissue elasticity.
An example of a method of obtaining tissue elasticity is disclosed in Japanese patent No. 5087378. This document discloses a frame interval adaptation device that interleaves ultrasound frames. According to this document, a set of beams 30 from one frame and a set of beams 32 from another frame are interleaved, and a beam from both frames is scanned during a frame interval 24 (FIG. 5, paragraph 0021). Using an average displacement history, so as to synchronize beam echo acquisition and cyclic displacement changes, acquiring displacement information is made easier by selecting ultrasound scanning frame intervals.
As another example, Japanese patent No. 5555286 discloses an ultrasound diagnostic device that calculates frame interval numbers according to displacement amounts that are inputted. This ultrasound diagnostic device calculates, based on movement amount (displacement) and force outputted from a displacement measurement unit 109 and a force measurement unit 110, distortion and elastic modulus of each point on a tomographic image, in order to calculate frame interval numbers according to distortion amounts.